A known technique of metal detection involves providing relative movement between a sample and detection apparatus comprising an oscillator coil and one or more detector coils. The oscillator coil is fed with oscillatory signals and these are inductively coupled to the detector coil(s). The presence of metal in the inductive path between the oscillator coil and the detector coil is indicated by a change in the signal derived from the detector coil. In one arrangement, one oscillator coil is provided with two detector coils spaced on either side thereof and equidistant therefrom. The detector coils are connected in series such that their induced E.M.F.s are opposed. Metal passing in the vicinity of such an arrangement will cause imbalance in the combined E.M.F. induced in the detector coils. If the coils are arranged such that the samples pass through the centre of each coil in turn, the detected signal will show an imbalance of one polarity as metal passes through the inductive path between the oscillator coil and one detector coil, and then an imbalance of the opposite polarity as the metal passes through the inductive path between the oscillator coil and the other detector coil.
A problem with this arrangement is that, because of thermal changes and changes in the properties of materials due to ageing, it is impossible to keep the E.M.F.s from the detector coils exactly in opposition. Consequently, a residual voltage appears which, if large enough, will cause any following circuits to be overloaded. Compensation for this voltage can be effected by subtracting from the residual voltage until it is balanced to zero. One of the problems with such balancing is that during the period that the detector circuit is responding to effect a measurement, the balancing mechanism will start to correct for the error signal which is produced as representing the measurement.